feat: Add a function for retrieving the window contents

This function is useful for testing the window contents in certain cases. In addition,
this means that we can now have reliable tests for softbuffer's actual functionality.

Signed-off-by: John Nunley <jtnunley01@gmail.com>
Co-authored-by: dAxpeDDa <daxpedda@gmail.com>

src/cg.rs

@@ -69,6 +69,11 @@ impl CGImpl {
             imp: self,
         })
     }
+
+    /// Fetch the buffer from the window.
+    pub fn fetch(&mut self) -> Result<Vec<u32>, SoftBufferError> {
+        Err(SoftBufferError::Unimplemented)
+    }
 }
 
 pub struct BufferImpl<'a> {